The failure to resolve meiotic intermediates as crossovers results in high levels of chromosome non-disjunction leading to sterility or inviable progeny. Many of the molecular details have been described for early events in meiotic recombination, but resolution of recombination intermediates into crossover (or non-crossover) products remains to be defined. Mutations in Drosophila mei-9 result in a dramatic decrease in meiotic crossovers and failure to repair mismatched DNA in recombination intermediates. mei-9 encodes a nuclease required for nucleotide excision repair (NER), and mutations in the human homologue of mei-9, XPF are associated with cancer predisposition. While the role of the MEI-9 endonuclease in NER is well understood, its function in meiosis remains unclear. Described herein is the proposal that meiotic Holliday junctions are cleaved by the MEI-9 endonuclease and that this cleavage is necessary for efficient resolution as a crossover. Additionally, it is suggested that the nicks introduced by MEI-9-dependent cleavage of Holliday junctions serve as an entry point for exonucleases to effect repair of mismatches in meiotic intermediates. This model allows one to make testable predictions about the arrangement of recombinant DNA on meiotic chromosomes. Molecular analysis will be performed on meiotic chromosomes derived from wild-type and mei-9 files. In addition, purified MEI-9 protein (and associated proteins) will be analyzed in vitro for the ability to cleave Holliday junctions.